Synchronizing a radio network with end user radio terminals

ABSTRACT

Synchronizing a Radio Network with End User Radio Terminals A Mobile Station that is able to receive GPS signals and compare the frequency of the GPS received time signal with a time signal from a network in order to determine the difference between the signals and communicate that difference back to the network.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/154,138 filed on May 21, 2002 entitled METHOD FORSYNCHRONIZING A RADIO NETWORK USING END USER RADIO TERMINALS, by GregoryB. Turetzky et al, which a claim to priority is made and is incorporatedby reference herein and further claims priority to U.S. ProvisionalPatent Application No. 60/292,774 filed on May 21, 2001 entitled “METHODFOR SYNCHRONIZING A RADIO NETWORK USING END USER RADIO TERMINALS”, byGregory B. Turetzky et al, which a claim to priority is made and isincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to Global Satellite System(GSS) receivers, and in particular to a method for synchronizing a radionetwork using end user radio terminals.

2. Related Art

Cellular telephony, including Personal Communication System (PCS)devices, has become commonplace. The use of such devices to providevoice, data, and other services, such as Internet access, provides manyconveniences to cellular systems users. Further, other wirelesscommunications systems, such as two-way paging, trunked radio,Specialized Mobile Radio (SMR) used by first responders, such as police,fire, and paramedic departments, have also become essential for mobilecommunications.

The Federal Communication Commission (FCC) has implemented a requirementthat Mobile Stations (MS), such as cellular telephones be locatablewithin 50 feet once an emergency call, such as a “911” call (alsoreferred to as “Enhanced 911” or “E911”) is placed by a given cellulartelephone. Such position data assists police, paramedics, and other lawenforcement and public service personnel, as well as other agencies thatmay need or have legal rights to determine the cellular telephone'sposition.

Currently, cellular and PCS systems are incorporating Global PositioningSystems (GPS) technology that uses GPS receivers in cellular telephonedevices and other wireless transceivers to meet the FCC locationrequirement.

Such data can be of use for other than E911 calls, and would be veryuseful for wireless network users, such as cellular and PCS subscribers.For example, GPS data may be used by the MS user to locate other mobilestations, determine the relative location of the mobile station user toother landmarks, obtain directions for the cellular user via internetmaps or other GPS mapping techniques, etc.

One significant problem with GPS receivers in a MS is that the GPSreceiver may not always have an unobstructed view of the sky causing thereceived signals to be very weak. Often, the receiver is unable todemodulate the Almanac or Ephemeris data, making it impossible todetermine the user's location or accurate GPS time. This problem may beaddressed by transmitting Ephemeris and/or Almanac data and GPS time tothe receiver over a communication network. A common feature ofcommunication networks is a large and variable transmission delay,making it difficult to transmit accurate (uncertainty less than 1millisecond) time.

The concept of locating a mobile unit by triangulating a set of rangesfrom either a set of fixed points (such as cellular transmitters) ormobile transmitters (such as GPS satellites) have a common requirementthat the time of transmission is known. This implies that the time atall transmitters must be common, or the differences known. In manysystems today, this information is not immediately available since thesystems are focused on data transmission rather than ranging. Therefore,there is a need in the art to overcome the problem of transmission delayin both synchronized and unsynchronized networks.

Code Division Multiple Access (CDMA)(TIA/IS-95B) networks use a GPS timereference standard at every base station, and all transmission framesare absolutely synchronized onto GPS time. Therefore, a Mobile Station,by observing particular transitions on frame, master frame or hyperframe, may predict absolute GPS time within tens of microseconds,including radio transmission delay and group delays inside the mobilestation or wireless handset.

Other classes of wireless networks, e.g., Time Division Multiple Access(TDMA), GSM, Analog Mobile Phone Systems (AMPS, TACS), DTV, etc., arenot synchronized onto GPS time. Still, the accuracy, precision andstability of the master clock used at the base stations is fairlystable, and slowly varies relative to GPS time. Hence, both the timeoffset and frequency drift are very stable compared to GPS time, and canbe monitored at relatively large intervals. However, any timinginformation derived solely from such a system has limited value, asthere is currently no way to derive absolute GPS time from it.

One solution that has been proposed is to locate stationary monitoringentities, called LMU (Local Measurement Units), which are in radiovisibility of several base stations (BS) in a given area. The LMUconsists of a wireless section and a GPS timing receiver. At intervals,they measure time offset and frequency drift of every base station inthe area, relative to GPS time. As one LMU can cover only a few BaseStations, the overlay monitoring network can become quite large andexpensive. It necessitates communication links between the LMU's and acentral network entity, which logs this information per BS, mergesinformation from different sources (if several LMU's monitor the sameBase Station), and delivers this information to a geolocation server iftime assistance has to be delivered to a particular MS in the BS'svisibility area. This requires several pieces of additional networkinfrastructure, as well as additional software and maintenance costs forthe network operator to enable such a feature. Thus, there is a need inthe art to eliminate the need for LMU's and the associated costs.

It can be seen, then, that there is a need in the art for delivering GPSdata in a wireless communications systems, including cellular and PCSsubscribers, in an efficient manner. It can also be seen that there is aneed in the art for GPS capable MS, such as wireless handsets. It canalso be seen that there is a need in the art to be able to aid the GPSreceiver to speed acquisition and for position determination. It canalso be seen that there is a need in the art to be able to aid the GPSreceiver to provide more precise position determination. It can also beseen that there is a need in the art for a large cellular system thatcan use and/or supply GPS information to cellular users for a number ofapplications, including E911 without the requirement of geographicallyproximate base stations.

SUMMARY

Approaches consistent with the present invention provide synchronizationof a radio network through the use of end user radio terminals. An enduser radio terminal, such as Mobile Stations (MS) having a GPS receiveris able to determine the relationship between the Base Station signaltiming events and GPS time, and to determine its clock frequency offset.This data may then be transferred to the Base Station (i.e. the network)for synchronization of the network. Other systems, methods, features andadvantages of the invention will be or will become apparent to one withskill in the art upon examination of the following figures and detaileddescription. It is intended that all such additional systems, methods,features and advantages be included within this description, be withinthe scope of the invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

The components in the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.In the figures, like reference numerals designate corresponding partsthroughout the different views.

FIG. 1 illustrates a typical OPS architecture.

FIG. 2 illustrates an implementation of synchronizing a radio networkwith end user radio terminals.

FIG. 3 is a diagram of the time tagging of GSM transmissions.

FIG. 4 is a diagram of GSM frames carrying GPS TOW.

FIG. 5 is a flow diagram of offset determination.

FIG. 6 is a flow diagram of a wireless handset using the offsetdetermined in FIG. 5.

DETAILED DESCRIPTION

In FIG. 1 a typical GPS architecture is shown. System 100 comprises GPSsatellite 102, which is illustrative of the constellation of GPSsatellites that are in orbit, a MS (i.e. wireless handset 104) which mayinclude a GPS receiver, a base station 106, a geolocation (server)service center 108, a geolocation end application 110, and a PublicSafety Answering Point (PSAP) 112. The Mobile Station (MS) 104, such asa wireless handset, Personal Digital Assistant (PDA), or similar mobiledevice may have location technology of the present invention and may useGPS technology in support of various MS device implementations of E911and geo-location services. The PSAP 112 and the geolocation endapplication 110 are included for reference only.

The GPS satellite 102 transmits spread spectrum signals 114 that arereceived at the wireless handset 104 and the geolocation server 108. Forease of illustrative purposes, the other GPS satellites are not shown,however, other GPS satellites also are transmitting signals that arereceived by the wireless handset 104 and the geolocation server 108. Ifthe wireless handset 104 receives strong enough spread spectrum signals114, the GPS receiver (not shown) in the wireless handset 104 mayautonomously compute the position of the wireless handset 114 as istypically done in the GPS system. However, unless they are in open skyenvironments wireless handsets 104 are typically not able to receivestrong enough spread spectrum signals 114 to autonomously compute theposition of the wireless handset 104, but can still communicate withbase station 106. Thus, base station 106 may communicate information viasignals 116 to wireless handset 104 to allow wireless handset 104 tocompute the location, or may transmit information from wireless handset104 to the geolocation server 108 to enable the geolocation server 108to compute the position of the wireless handset 104. If the base station106 is transferring information to the wireless handset 104 to allow thewireless handset 104 to compute position, it is called “wireless aidedGPS” or “MS Based,” whereas when the base station 106 transfersinformation from the wireless handset 104 to the geolocation server 108for the geolocation server 108 to compute the position of the wirelesshandset 104, it is called “network-centric GPS” or “MS Assisted.”

Geolocation server 108 may also communicates with geolocationapplication 110 via signals 118 and with PSAP 112 via signals 120. Thesesignals 118 and 120 may either be via wireless links, such as cellular,WiFi, Blue Tooth, to name but a few, or may be through the landlinenetwork, such as PSTN, Ethernet, or other such wired networks, to namebut a few.

If it is a cellular telephone, for example, the wireless handset 104 mayinclude a typical wireless handset section that performs the callprocessing (CP) function, and a GPS section for position computation,pseudorange measurement, and other GPS functions. A serial communicationlink, or other communication link, performs the communications betweenthe CP section and the GPS section. A collection of hardware lines maybe utilized to transmit signals between the CP and GPS section. In yetanother implementation, both the CP and GPS sections may sharecircuitry.

If the MS 104 has the ability to compute GPS position, it gets GPS timefrom the GPS signal, and is able to calculate the offset between GPStime and the cell site clock. This is true whether or not the GPSportion of the MS 104 received assistance data from the geolocationservice center 108. In unsynchronized networks, each cell site clockwill have a different offset from GPS time, necessitating the pairing ofcell site identifiers with the measured offset. In some wireless handsetdesigns, the frequency error of the base station clock may also becomputed.

The offset and frequency error may then be stored in the phone, and/ortransmitted to the network (via signals 116) for storage in a database(possibly contained in the geolocation service center 108). Each time awireless handset goes through that cell, the offset and error may beupdated. If it is not possible to make a direct measurement of basestation frequency error, then multiple clock-offset measurements may beused to determine drift rates.

Non-network related storage that is capable of being accessed via a datalink such as SMS or GPRS may also be used such that independent serviceproviders could store and forward time assistance information to otherwireless handset units independent of the network.

This concept may also be used in conjunction with other localizednetworks like Nextel, SMS, FRS, etc. where a group of wireless handsetsor mobile communication devices may help each other to determinelocation. For example, where a wireless handset gets a fix, thatwireless handset can transmit offset information, or transmit otherinformation via a non-cellular network, such as SMS, CB bands, WiFi,Blue Tooth, or whatever, to other wireless handsets that use thatnetwork, or are part of a group of devices used by the same company.

If the MS 104 lacks the ability to compute GPS position, it may capturesimultaneous events from the GPS signals and the Base Station signals,and send them via signals 116 to a server, which is able to compute GPSposition of the MS 104. After such computation, the server will be ableto determine precise GPS time, and compute the offset (and drift)between GPS time and the clock in the Base Station. This information maythen be transmitted via signals 116 to other MS 104 devices to assisttheir acquisition of GPS signals, whether or not those MS devices havethe ability to compute their own GPS position.

Turning to FIG. 2, an implementation of synchronizing a radio networkusing end user radio terminals is shown. System 100 has a set of GPSsatellites (illustrated by 102), a base station 106, a geolocationservice center 108 and two wireless handsets 104 and 105.

As described earlier, wireless handset 104 receives signals fromsatellites 102 and either computes a GPS position locally, or transmitsvia signals 116 sufficient information to allow a server, such as thegeolocation service center 108, to compute the position. Concomitantwith computing the GPS position, a controller (not shown) in thewireless handset 104 or the geolocation service center 108 or some otherdevice (not shown), determines the time offset and/or drift between GPStime and the clock in the base station 106.

Wireless handset 105 is illustrative of a wireless device containing aGPS receiver which requires knowledge of the clock offset and/or driftof the base station 106 clock in order to acquire satellite 102 signalsand produce a GPS position fix. Depending on the type of network and itsdesign, wireless handset 105 may receive the required data from wirelesshandset 104 directly via signals 202, from base station 106 via signals204, or from the geolocation service center via signals 116 and 204 insequence. Other sources of this information may include non-networkdevices (not shown) that may be implemented by independent serviceproviders.

In another implementation, wireless handset 105 and wireless handset 104may be the same wireless handset, used at different times. Wirelesshandset 104 may compute the clock offset and drift at one time, then beturned off and forget the previously computed data. Upon beingre-powered, wireless handset 104 may require this data and may retrieveit (or a more recently computed value) from the base station 106, thegeolocation service center 108 or some other source.

Alternatively, wireless handset 104 may compute the clock offset and/ordrift of the clock in base station 106, then be turned off, but storethe previously computed data. Upon being re-powered, the wirelesshandset may recall the data from its own memory without making use ofany external data store. In some cases, this may eliminate the need fortimekeeping in the MS when the MS is powered off which may increasebattery time between charging.

The wireless handset may also build up a database of offsets computedfor several different base stations, and since the base station clocksare stable for long periods, that information is useful when thewireless handset returns to that base station. Thus, when the mobile GPSreceiver in a wireless handset or similar enabled device returns to aknown cell site at a later time, the mobile GPS receiver already knowsthe offset between the cell site clock and GPS time, making a TTFFshorter for that mobile GPS receiver.

In FIG. 3, the time tagging of GSM transmissions is shown. A GSM networkhas been chosen for illustration. Other networks will have a similarimplementation. This time-tagging is essential to the process ofmeasuring the offset between GPS time and “network” time.

The CP section of the wireless handset 104 that has a valid GPSsolution, generates a time mark 110 that may be implemented as ahardware pulse that the GPS receiver in the wireless handset tags withits own clock that has a known relation with the GPS system time thatincludes a “Time of Week” (TOW) portion. The CP section may also send amessage to the GPS receiver identifying the GSM frame and bit numberassociated with the time mark, and the base station being used, as shownin Table 1. In the current implementation, a GPS time tag for thereceived GSM bit may be used for time tagging of the GSM transmission.By subtracting the time lag for transmission between the base stationlocation and the wireless handset location, the wireless handset knowsthe GPS time when the GSM bit left the transmitting antenna. Thesubtraction may be done in the wireless handset or the geolocationservices center (i.e. a relocation server), but the server is used inthe current embodiment.

In yet another implementation, the wireless handset 104 may measure thefrequency difference between the GPS clock and the call processing clock(provided the GPS clock and call processing clocks are not the sameclock). The GPS receiver in the wireless handset 104 may already havethe ability to measure the frequency difference between its clock andthe GPS system frequency standard. Similarly, the wireless handset mayalso already have the ability to measure the frequency differencebetween its call processing clock and the frequency received from theRadio Network transmitter located at a base station. Thus, all thecomponents may be incorporated into a wireless handset to measure thefrequency difference between the GPS system frequency standard and thewireless network transmitter frequency and may be located in the CPsection of the wireless handset or the GPS receiver section depending onthe design and implementation of the wireless handset.

Table 1 contains the information supplied by the CP section to accompanythe time mark:

TABLE 1 Name Description Base Station Unique ID for current base stationCP_GSM_Frame GSM Frame Number CP_GSM_BIT GSM Bit NumberTime_mark_uncertainty Probable error between time mark and received bitedge (1 sigma)

The geolocation server 108 may receive a number of parameters 112 fromthe wireless handset 104 including, but not limited to a GSM bitidentifier, the associated GPS TOW and base station ID, position data,and frequency error. Once the clock offset and frequency difference isdetermined, a Kalman Filter or other estimation method may be used tomodel the wireless network's transmitter clock. In otherimplementations, the transmitter clock may be adjusted to minimize theerrors. Such knowledge of the transmitter clock frequency and timeerror, enables better performance of the GPS receiver's TTFF, energyusage and position accuracy.

At a later time, the geolocation service center may propagate the storedtime-tagged GSM frame/bit information to an approximate current time.This propagated time may then be transmitted to an acquiring wirelesshandset that does not currently have a GPS solution as described below.

Turning to FIG. 4, a diagram of GSM frames carrying GPS TOW. Thisfunctionality provides for accurate GPS time to a wireless handset 104that does not yet have a GPS position. It also illustrates the method bywhich the present invention compensates for network delays.

When the wireless handset 104 requests aiding from the geolocationservice center 108, a message from the server to the wireless handset issent. The message identifies the GPS time with a specific GSM frame/bit,identified as “GSM Bit Y” in the figure. The server creates this messagefrom earlier measurements made by this, or other, wireless handsets asdescribed above. When the message is received at the wireless handset104, the CP section of the wireless handset 104 generates a time markaligned with a current GSM frame/bit, identified as “GSM Bit X” in thefigure. The CP section may also send a message to the GPS receiveridentifying the GSM frame and bit number associated with the time mark,and the base station being used, as shown in Table 1. The GPS receiverwill then propagate the GPS time from the bit identified in the message(Y) to the bit that is aligned to the time mark (X), using nominal, (orcorrected, if clock drifts are available) frame rates, thus compensatingfor the network delay and geolocation service center 108 time estimationerrors. Because the wireless handset 104 location is unknown, there isan unknown transmission delay from the base station 106 to the wirelesshandset 104. This delay presents an unavoidable error in the receivedGPS time, but is limited by the typically small sizes of cellular radiosites.

In Table 2, an example of one possible message sent from the geolocationserver 108 to the GPS receiver in an acquiring wireless handset 104 isthe following:

TABLE 2 NAME Description Units Notes gps_time_tag VLMU_GPS_Week GPS WeekNumber Weeks These are shown as VLMU_GPS_TOW GPS Time of Week Usec GPSTOW in FIG. 4. freq VLMU_Freq_Error Base Station Freq. Error Nsec/secThis is ‘freq’ in FIG. 4. List_of_meas_uncertainties VLMU_Time_AccuracyUncertainty of GPS time Usec VLMU_Freq_Err_Acc Uncertainty of ClockError Nsec/sec network_reference_time VLMU_GSM_Frame GSM Frame NumberNone This is bit Y in FIG. 4 VLMU_GSM_Bit BSM Bit Number NoneThe items in Table 2 may be repeated once for each base stationidentified in a data structure such as a neighbor list that identifiesbase stations near the current base station in the wireless network. Insome implementations, the CP section of the wireless handset 104 mayfilter the list of base stations and only provide data for the servingbase station.

Using the data items in Table 1 and Table 2, the algorithms used toconvert time-tagged GSM frames to precise GPS time employed in thecurrent implementations are:

CP_Bits = CP_GSM_Frame * 1250 + CP GSM_Bit VLMU_Bits = VLMU_GSM_Frame *1250 + VLMU_GSM_Bit DeltaGSM = CP_Bits-VLMU_Bits IF deltaGSM <−2710000 * 1250 deltaGSM += 2715648 * 1250 ELSEIF deltaGSM > 2710000 *1250 DeltaGSM −= 2715648 * 1250 ENDIF DeltaTime = deltaGSM *SecPerGSMBit/(1+VLMU_Freq_error*1e⁻⁹) GPS_TOW = VLMU_GPS_TOW +deltaTime * 1e⁶ GPS_Week = VLMU_GPS_Week IF (GPS_TOW >= 604800 * 1e⁶){GPS_TOW −= 604800 * 1e⁶ GPS Week++ } ELSEIF (GPS_TOW < 0 {GPS_TOW +=604800 * 1e⁶ GPS_Week−− }time_uncertainty=VLMU_Time_Accuracy+time_mark_uncertainty.

In FIG. 5, a flow diagram 500 of offset determination is shown. Awireless handset 104 receives a GPS signal 114 at a GPS receiver in step500. The wireless handset 104 also receives a communication signal 116from the wireless network in step 504 that contains timing information.The controller then determines the time offset and/or drift between theclock at base station 106 sent in the received communication signal 116and the GPS time sent in GPS signal 114 in step 506. The offset may thenbe sent back to the current base station 508.

Turning to FIG. 6, a flow diagram 600 of a wireless handset using offsetdetermined in FIG. 5 is illustrated. A wireless handset 104 requestsaiding from the geolocation server 108 in step 602, a message from thegeolocation server 108 to the wireless handset is sent, step 604. Themessage identifies the GPS time with a specific GSM frame/bit,identified as “GSM Bit Y” in FIG. 4. The geolocation server 108 createsthis message from earlier measurements made by this, or other, wirelesshandsets. When the message is received at the wireless handset 104, theCP section of the wireless handset 104 generates a time mark alignedwith a (possibly) different GSM frame/bit, identified as “GSM Bit X” inFIG. 4, see step 606. In step 608, the CP section may also send amessage to the GPS receiver identifying the GSM frame and bit numberassociated with the time mark, and the base station being used, as shownin Table 1. The GPS section of the wireless handset 104 propagates theGPS time from the bit identified in the message to the bit associatedwith the time mark, thus compensating for network delays and time errorscaused by the geolocation service center server 108 in step 610. Becausethe wireless handset 104 location is unknown, there is an unknowntransmission delay from the base station 106 to the wireless handset104. This delay presents an unavoidable error in the received GPS time,but is limited by the typically small sizes of cellular radio sites.

The flow diagrams in FIG. 5 and FIG. 6 may be implemented in software orhardware or a combination of software and hardware. The software may bepresented on a signal-bearing medium that contains machine-readableinstructions such as magnetic tape, compact disc, paper punch cards,smart cards, or other optical, magnetic, or electrical digital storagedevice. A controller may execute the software presented on thesignal-bearing medium. Examples of a controller may include amicroprocessor, digital signal processor, digital circuits configured tofunction as a state machine, analog circuits configures to function as astate machine, a combination of any of the above configured to executethe programmed instructions, such as presented on the signal-bearingmedium.

The foregoing description of an implementation has been presented forpurposes of illustration and description. It is not exhaustive and doesnot limit the claimed inventions to the precise form disclosed.Modifications and variations are possible in light of the abovedescription or may be acquired from practicing the invention. Forexample, the described implementation includes software but theinvention may be implemented as a combination of hardware and softwareor in hardware alone. Note also that the implementation may vary betweensystems. The claims and their equivalents define the scope of theinvention.

1. A wireless mobile communication device, comprising: a transmitter inthe wireless mobile communication device to send a request message thatrequests aiding information from a geolocation server; a receiver in thewireless mobile communication device in receipt of a response messagefrom the geolocation server that is other than a cellular base station,wherein the response message identifies absolute time with a firstframe/bit number (bit Y); a call processing section coupled to thereceiver that generates a timing mark aligned to a second frame/bitnumber (bit X), wherein the second frame/bit number (bit X) is differentfrom the first frame/bit number (bit Y); and a Global Positioning System(GPS) receiver in the wireless mobile communication device in receipt ofa notification message from the call processing section that identifiesthe second frame/bit number (bit X) aligned to the timing mark, wherethe GPS receiver propagates the absolute time from the first bit (bit Y)identified in the response message to the second bit (bit X) aligned tothe timing mark that enables the GPS receiver to compensate for networkdelay and time errors caused by the geolocation server.
 2. The wirelessdevice of claim 1, where each of the first frame/bit number (bit Y) andthe second frame/bit number (bit X) is a GSM frame/bit number.
 3. Thewireless device of claim 1, where the network delay comprisescompensation for time estimation errors of the geolocation server.
 4. Amethod for synchronizing a wireless mobile communication device,comprising: sending by a transmitter in the wireless mobilecommunication device a request message that requests aiding informationfrom a geolocation server; receiving at a receiver in the wirelessmobile communication device a response message from the geolocationserver that is other than a cellular base station, wherein the responsemessage identifies absolute time with a first frame/bit number (bit Y);generating in a call processing section that is coupled to the receiver,a timing mark aligned to a second frame/bit number (bit X), wherein thesecond frame/bit number (bit X) is different from the first frame/bitnumber (bit Y); receiving at a Global Positioning System (GPS) receiverin the wireless mobile communication device a notification message fromthe call processing section that identifies the second frame/bit number(bit X) aligned to the timing mark; propagating the absolute time fromthe first bit (bit Y) identified in the response message to the secondbit (bit X) aligned to the timing mark; and compensating for networkdelay and time errors caused by the geolocation server in the GPSreceiver.
 5. The method of claim 4, where each of the first frame/binumber (bit Y) and the second frame/bit number (bit X) is a GSMframe/bit number.
 6. The method of claim 4, where compensating fornetwork delay comprises compensating for time estimation errors of thegeolocation server.